1. Field of the Invention
The present invention relates to a color cathode ray tube to be used for e.g. a display for a television broadcast receiver (hereinafter referred to as a television) or a computer, and a glass bulb to be used for such a cathode ray tube.
2. Discussion of the Background
Firstly, the construction of a color cathode ray tube will be described referring to the attached drawings. FIG. 1 is a partially cross-sectional view of the entirety of the color cathode ray tube. FIG. 2 is an enlarged view of FIG. 1 at a portion S including the sealing portion and its vicinity. Here, in the present invention, a cathode ray tube is meant for a color cathode ray tube unless otherwise specified.
The envelope of the cathode ray tube 1 is constituted by a glass bulb 2 which basically comprises a panel 3 for displaying picture images, a funnel-shaped funnel 4 sealingly bonded to the panel 3 and a neck 5 accommodating an electron gun 17. The panel 3 is constituted by an approximately rectangular face portion 7 constituting a picture image-displaying screen and a skirt portion 6 extending in a direction substantially perpendicular to the face portion 7 from its periphery via a blend R portion 9.
An explosion proof reinforcing band 8 is wound around the circumference of the skirt portion 6 to maintain the panel strength and to prevent scattering upon breakage. On the inner surface side of the face portion 7, a phosphor screen 12 which emits fluorescence by electron beam bombardment from an electron gun 17 and an aluminum film 13 to reflect the fluorescence emitted from the phosphor screen 12 towards the rear side of the cathode ray tube (towards the funnel 4 side), to the front side (to the face 7 side), are laminated, and a shadow mask 14 which regulates the position for electron beam bombardment, is further provided. The shadow mask 14 is fixed to the inner surface of the skirt portion 6 by stud pins 15. Further, A in FIG. 1 indicates a tube axis connecting the center axis of the neck 5 and the center axis of the panel 3.
Such a panel 3 is sealingly bonded to a seal edge portion 16xe2x80x2 of the funnel 4 by a sealing material such as a solder glass provided at the seal edge portion 16 corresponding to the end portion of the skirt portion 6, whereby a sealing portion 10 is formed.
The glass bulb 2 for a cathode ray tube having the above construction, is used as a vacuum vessel, whereby atmospheric pressure is exerted to the outer surface. The glass bulb is in unstable deformed state due to an asymmetrical shape as is different from a spherical shell, and a stress is exerted over a relatively wide range (a stress formed when the glass bulb is vacuumized, will hereinafter be referred to as a vacuum stress). In such a state that a high tensile vacuum stress is applied to the outer surface, a delayed fracture may take place due to the effect by moisture in the atmosphere, which may cause decrease in safety and reliability.
The face portion 7 as a portion which displays picture images has the highest flatness in the cathode ray tube and thereby has a low rigidity, and it is most significantly deformed when the inside of the cathode ray tube is depressurized and an atmospheric pressure is applied thereto. Further, the face portion 7 is supported by the blend R portion 9 having a high rigidity, whereby a high tensile vacuum stress is likely to generate in the vicinity of the blend R portion 9 along with the deformation of the face portion 7. Further, the deformation of the face portion 7 functions as a force to deform the skirt portion 6 towards the outside via the blend R portion 9, and accordingly a high tensile vacuum stress is generated also at the sealing portion 10.
However, the sealing portion sealingly bonded by means of a sealing material has the lowest allowance against the tensile vacuum stress in the glass bulb, and the allowable stress at the sealing portion becomes lower when the accuracy of the flatness at the sealing surface between the panel and the funnel is low.
A television employing a cathode ray tube has a demerit of being heavy as compared with a plasma display and a liquid crystal display, whereby weight reduction of a glass bulb has been desired. Further, in recent years, a cathode ray tube having a face portion having a higher flatness has been desired to decrease distortion of picture images as far as possible to improve visibility. However, by making the face portion flat, asymmetry of the glass bulb shape increases, and the glass bulb is in a further unstable deformed state, whereby tensile vacuum stress generated to the respective portions tends to increase. In addition, the amount of glass used tends to decrease as compared with conventional ones due to weight reduction, whereby a higher deformation energy tends to be accumulated on the glass bulb, thus increasing possibility of destruction.
Accordingly, if the panel thickness is made thin and the face portion is made flat at the same time to accomplish such weight reduction, a tensile vacuum stress generated at the face portion will significantly increase as described above. To overcome the above problem, tempering methods to produce a compressive stress to the panel surface have been developed.
Heretofore, as a means to reduce the weight of the glass bulb for a cathode ray tube, it has been practically proposed to form a compressive stress layer on the surface of a panel in a thickness of about ⅙ of the glass by means of e.g. a physical tempering method, as disclosed in Japanese Patent No. 2904067. However, it is impossible to uniformly quench a panel or funnel having a three dimensional structure and a non-uniform thickness distribution. Consequently, due to the non-uniform temperature distribution, a large tensile residual stress will be formed together with the compressive stress, whereby the compressive stress is rather limited to a level of 30 MPa at best, and it has been impossible to produce a large compressive stress. Namely, when a physical tempering method is employed, the weight reduction of the glass bulb is limited, since the compressive stress which can be produced, is relatively small.
On the other hand, it is known to reduce the weight by tempering the surface of a glass bulb by an ion-exchange method. This method is a method wherein certain alkali ions in glass are substituted by ions larger than the alkali ions at a temperature of not higher than the distortion point, and a compressive stress layer is formed on the surface by the volume increase. For example, it can be accomplished by immersing a strontium/barium/alkali/alumina/silicate glass containing from about 5 to about 8% of Na2O and from about 5 to about 9% of K2O, in a molten liquid of KNO3 at about 450xc2x0 C. In the case of such ion-exchange method, a large compressive stress at a level of from 50 to 300 MPa can be obtained, and it is advantageous for the weight reduction over the physical tempering in that no necessary tensile stress will be formed.
The ion-exchange method is usually carried out in the process of panel production, i.e. it is carried out after press molding and polishing, whereby a high compressive stress can be produced to the face portion and the skirt portion. However, the sealing portion is provided in such a manner that after e.g. shadow mask is attached to the inside of the panel, the seal edge portion of the panel and the seal edge portion of the funnel are put together and welded by means of a sealing material such as a solder glass, whereby no compressive stress can be produced by means of an ion-exchange method, and accordingly the difference in strength between the face portion and the seal portion tends to further widen.
On the other hand, the face portion to which a high compressive stress is produced by an ion-exchange method, can tolerate a high tensile vacuum stress as compared with a conventional one, and consequently, the face portion can significantly be made thin, which contributes to weight reduction. However, if the face portion is made thin as far as possible based on the compressive stress value produced by the ion-exchange method, the distortion amount of the face portion tends to increase, whereby the tensile vacuum stress to be generated at the sealing portion may further increase.
The sealing portion is formed by sealingly bonding the panel and the funnel by means of a sealing material as described above. A baked product of a sealing material such as a solder glass has a strength of from 60 to 70% as compared with the panel, and the strength at the sealing portion is weakest in the glass bulb due to such strength of the sealing material. Further, no compressive stress is produced to the sealing portion by an ion-exchange method.
Consequently, if a tensile vacuum stress is generated in the glass bulb employing a thin panel having a compressive stress layer formed thereon by an ion-exchange method, although the tensile vacuum stress is allowable for the face portion of the panel, it may reach the upper limit of the allowance at the sealing portion, and accordingly a face portion can not be made thin as far as possible, thus inhibiting weight reduction.
The present invention has been made in view of the above problems, and it is an object of the present invention to provide a glass bulb for a cathode ray tube wherein weight reduction and/or flattening of the face portion can be achieved by producing a high compressive stress by an ion-exchange method without decreasing the strength at the sealing portion, and to provide a cathode ray tube employing said glass bulb for a cathode ray tube, having a high safety.
To accomplish the above object, the present invention provides a glass bulb for a color cathode ray tube, which comprises a panel, a funnel connected to the panel and a neck, the panel comprising an approximately rectangular face portion and a skirt portion constituting a side wall of the face portion and having a seal edge portion at the end portion, wherein such a compressive stress "sgr"c that 50 MPaxe2x89xa6|"sgr"c|xe2x89xa6250 MPa is produced by an ion-exchange method to at least one of short axis end portions and long axis end portions on the outer surface of the face portion; the average thickness t=(tc+tmax)/2 of the face portion represented by the central thickness tc of the face portion and the maximum thickness tmax of the face portion, and the thickness tse at the seal edge portion, satisfy the relation t/tsexe2x89xa61.4; and the maximum value "sgr"VTmax of the tensile stress generated at the face portion when vacuumized is 20 MPaxe2x89xa6"sgr"VTmax less than 200 MPa. The present invention further provides a color cathode ray tube employing the above glass bulb.
In the glass bulb for a color cathode ray tube, the compressive stress "sgr"c is more preferably 50 MPaxe2x89xa6|"sgr"c|=xe2x89xa6200 MPa. It is more preferred that the compressive stress is 80 MPaxe2x89xa6|"sgr"c|xe2x89xa6150 MPa and the maximum value "sgr"VTmax of the tensile stress is 20 MPaxe2x89xa6"sgr"VTmaxxe2x89xa6100 MPa, whereby a high industrial productivity can be obtained. Further, in the glass bulb for a color cathode ray tube of the present invention, the proportion of the average thickness t of the face portion to the thickness tse at the seal edge portion, i.e. t/tse is preferably 0.5xe2x89xa6t/tsexe2x89xa61.0, whereby further weight reduction becomes possible without generation of a high tensile vacuum stress at the sealing portion.